That's easy. When you compose a tweet, you send it to the ampersands separated by a space.. for example, with your list.. In the message box, list the ampersands followed by the message. However, you're only allowed so many characters per message (it counts them down for you as you type).. you may have to send one message to 3-4 people with a short message.
@LauraPrepon @marisolnichols @Chill_EB @KateCeberano @4everBrandy @EthanSuplee <message>
If you go on to someone's Twitter account, you will find their ampersant & account name, i.e., Jenna Elfman is @JennaElfman.
https://twitter.com/JennaElfman
FYI, the "user name" after the ampersand doesn't always follow the name on the account, i.e., when Jason Lee was tweeting (he's since deleted his account), his ampersand was @ThatJasonLee.